Method of applying nonpermanent decorative coating to high-strength,low alloy steel



METHOD OF APPLYING NONPERMANENT DEC- ORATIVE COATING TO HIGH-STRENGTH, LOW ALLOY STEEL Joseph Bigos, Plum Borough, and Bruno M. Perfetti, Jeanette Borough, Pa., assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed June 13, 1966, Ser. No. 556,908

Int. Cl. B44d 1/34; C09d 3/26; C23c 11/10 US. Cl. 148-6.35 3 Claims ABSTRACT OF THE DISCLOSURE The surface of certain high-strength low-alloy steels weathers through oxidation to a decorative dark earthy color, but the natural weathering process takes several years. The steel does not have its full attractiveness until the weathering is completed. The invention provides a method of treating the steel to impart the same appearance initially that it acquires later on weathering. The steel is coated with a paint composition which degrades gradually and uniformly at approximately the same rate as the steel weathers, but does not interfere with weathering. The pigment of the paint matches the color of the weathered steel.

This invention relates to a method of coating steels which acquires a protective and decorative oxide patina on weathering, to compositions for coating such steels, and to the coated product.

The surface of certain high-strength low-alloy steels weathers through oxidation to a dark earthy color. The oxide patina which forms is extremely dense and adherent, and inhibits further corrosive attack. It also has an attractive appearance. However, the weathering process may take several years, and in the meantime a structure in which such steel is exposed to view does not have its full attractiveness. As far as we are aware, the weathering process cannot be hastened, but must take place only through natural exposure.

An object of our invention is to provide a method of coating scale-free steel of the foregoing type to simulate the appearance it acquires after weathering.

A further object is to provide a method of achieving this appearance Without hindering natural weathering processes, and with a coating which degrades gradually and uniformly in approximately the same time as required for weathering.

A further object is to provide improved coating compositions which have a low pigment-to-binder ratio and which degrade by gradual uniform erosion resembling chalking, whereby they give a steel surface a weathered appearance without inhibiting the natural weathering.

A further object is to provide a coated steel product which simulates the appearance of weathered steel, but which subsequently weathers normally.

According to our invention, we first pickle or blastclean the surface of the steel until it is scale-free. We coat the scale-free surface with a composition which consists of a vehicle binder, a pigment, and a thinner or solvent. The binder is a film-forming, adherent, resinous, organic substance susceptible to degradation by ultraviolet light and weathering to a chalky, porous material when applied to a metal substrate as a film of 0.2 to 2.0 mil thickness. Our preferred binder is a water-soluble polymerized soya oil, but we also have used successfully shellac solutions, styrenated alkyd solids and oleoresinous varnishes. We use appropriate pigments which match the color the steel acquires later after it has weathered, for example raw or burnt umbers, red or brown iron oxides, raw or burnt siennas, carbon black, or combinations United States Patent "ice thereof. We use conventional thinners in which the binder is soluble. With our preferred soya oil binder, we use water and ammonium hydroxide as a thinner. With other binders we can use alcohol, toluene or mineral spirits.

Our composition has a much lower pigment-to-binder ratio than conventional coating compositions, where the objective is to produce lasting coatings. We use a minimum ratio of pigment to binder of about 0.1 by weight and a maximum ratio of about 0.4, or preferably a ratio between about 0.20 and 0.25. A ratio below our minimum does not furnish sufiicient pigment to color the steel, while a ratio above our maximum inhibits natural weathering. These ratios are in contrast to a minimum ratio of about 0.8 used in conventional paints, or more often 1.0 to 2.0. We include sufiicient thinner to produce a composition which is readily applied as a thin coating in the range of 0.2 to 2.0 mils thickness. The proportion of thinner is about 35 to percent by weight of the total composition. Specific examples of compositions within our invention are as follows:

EXAMPLE I The binder was a water-reduced polymer derived from soya oil. We synthesize the binder by forming an ammoniumr salt of a polycarboxylated glyceride (maleated soya bean oil). Thus the binder is miscible with water. The composition was as follows:

Parts by weight Polymerized soya oil solids (as 50% solids solution in water) 20 Water 20 Burnt umber The pigment-to-binder ratio was 0.20. We mixed the pigment and binder in a bead and paddle mill. We added just sufficient NH OH to reduce the soya oil binder to a workable viscosity. We transferred the mixture to a pebble mill and ground it to proper fineness.

EXAMPLE II The binder was a shellac solution. The composition was as follows:

Parts by weight Shellac solids 24 Denatured alcohol 24 Burnt umber 3 Brown iron oxide 3 The pigment-to-binder ratio was 0.25. We added the pigment slowly to the solution of shellac and alcohol as we stirred it in a bead and paddle mill. We continued stirring until all the pigment was visibly wetted and dispersed. We strained the composition and transferred it to a pebble mill and ground it to proper fineness.

EXAMPLE III The binder was a styrenated alkyd synthetic resin formed by modifying a conventional alkyd resin through copolymerization with styrene monomer. We form the alkyd resin by polymerizing a polyhydric alcohol, such as glycerol, with a difunctional anhydride, such as I phthalic anhydride, and a long chain unsaturated vegetable oil, such as linseed or soya bean oil. We add styrene monomer to this polymer. Styrene alkyd resins dry more rapidly in air and produce harder films than conventional drying alkyd resins. We can add a drier to this binder to monomer to this polymer. Styrene alkyl resins dry more film. The drier can be one of the common metallic driers used in paints and varnishes, such as lead, cobalt, manganese, zinc, calcium, zirconium and rare earth driers. Driers are solutions in mineral spirits of heavy metal soaps of organic acids which form stable mixtures with most alkyd and oleoresinous binders. The composition of this example was as follows:

Parts by weight Styrenated alkyd solids containing drier 25 Toluene 25 Medium boiling range aromatic solvent 25 Burnt umber 2.4 Brown iron oxide 2.4

Diatomaceous silica 0.2

The pigment-to-binder ratio was 0.20. We added pigment to approximately half the binder and ground the mixture in a pebble mill to proper fineness. Thereafter we added the remainder of the binder with thorough mixing.

EXAMPLE IV The binder was a rosin-ester type varnish prepared through high-temperature copolymerization of unsaturated, long-chain vegetable oils, such as oiticica oil, with rosin ester resins. Rosin ester resins are the reaction products of the polymerization of rosin and polyhydric alcohols, modified by incorporating para-substituted phenols. Typical para-substituted phenols are bis-phenol, p-tertiary butyl phenol, p-tertiary amyl phenol, and pphenyl phenol. Rosin ester resins also may be modified by formation of adducts with the resin prior to polymerization With the polyhydric alcohol. These adducts are prepared by reacting rosin with such difunctional materials as maleic anhydride or fumaric acid. The composition of this example Was as follows:

Parts by weight Oleoresinous varnish solids 24 Mineral spirits 40 Brown iron oxide 5 Burnt umber 1 Cobalt naphthenate drier 0.1 Manganese naphthenate drier 0.1 Diatomaceous silica 0.3

The pigment-to-binder ratio was 0.25. We stirred the pigment into the varnish solution containing approximately half the mineral spirits solvent, transferred the mixture to a pebble mill, and ground it until all the pigment was properly dispersed. We added the remaining solvent, strained and added the drier while agitating.

We can apply any of the foregoing compositions to a pickled or blast-cleaned scale-free surface of steel as a thin coating, approximately 1 mil in thickness. We may use any conventional technique for applying the coating. The burnt umber and brown iron oxide pigments simulate the color which a high-strength low-alloy steel acquires after the weathering process is complete. The binder enables the coating to degrade gradually and uniformly at approximately the same rate as the Weathering process proceeds, but does not interfere with natural weathering. Therefore our method assures that the steel has the desired attractiveness from the beginning that it subsequently acquires through natural processes.

While we have described only certain preferred ways of practicing our invention, it is apparent other modifications may arise. Therefore we do not wish to be limited by the disclosure set forth, but only by the scope of the appended claims.

We claim: 1. A method of treating a high-strength low-alloy steel body of the type which acquires a protective and decorative oxide patina on its surface after it weathers for a protracted period, said method comprising:

applying to a scale-free surface of the body before it has Weathered a coating of a composition which consists of a pigment, a water soluble polymerized soya oil and water and ammonium hydroxide as thinner binder,

the pigment-to-binder ratio being about 0.1 to 0.4 by

weight;

said pigment matching the color which the body acquires later after weathering, and being selected from the group consisting of raw and burnt umbers, red and brown iron oxides, raw and burnt siennas, carbon black, and combinations thereof;

said binder being a film-forming adherent, resinous,

organic substance;

said solvent constituting about 35 to percent by weight of the composition;

said coating having a thickness of 0.2 to 2.0 mils;

weathering the coated body by prolonged natural exposure to roduce the oxide patina without interference from said coating; and

degrading the coating during the weathering process by gradual and uniform erosion in approximately the same period as required for completion of the Weathering process;

whereby the body initially has an appearance similar to that which it acquires later after weathering.

2. A method as defined in claim 1 in which said composition has a pigment-to-binder ratio between about 0.20 and 0.25 by weight.

3. A structure having incorporated therein steel members prepared by the method defined in claim 1.

References Cited UNITED STATES PATENTS 3,036,976 5/1962 Sanderson 117-132 3,035,926 5/1962 Larrieu 10614 3,067,044 12/1962 Johnston et al. 10614 3,260,609 7/1966 Reeser 10614 3,311,529 3/1967 Chisholm 106-14 ALLAN LIEBERMAN, Primary Examiner U.S. Cl. X.R.

UNITEI) STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,502, 512 March 24 1970 Joseph Bigos et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 67, "monomer to this polymer. Styrene alkyl resins dry more" should read accelerate conversion of a liquid coating to a dry paint Column 4, line 14, cancel "and water and ammonium hydroxide as thinner" and insert the same after "binder" in line 15, same column 4.

Signed 'and sealed this 15th day of December 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Ir.

Commissioner of Patents Attcsting Officer 

